Preparation of linear polycarbonates from cyclic oligomer compositions using zwitterionic catalyst

ABSTRACT

Cyclic polycarbonate oligomers are converted to linear polycarbonates by the catalytic action of a zwitterionic compound containing positively charged nitrogen and negatively charged oxygen. The polymerization reaction is slow, and therefore particularly useful in such operations as resin transfer molding.

This invention relates to the preparation of linear polycarbonates andsimilar condensation polymers, and more particularly to a method fortheir preparation from cyclic oligomer compositions.

The conversion of low molecular weight cyclic aromatic carbonatepolymers to linear polycarbonates is known. Reference is made, forexample, to the following U.S. Pat. Nos.:

    ______________________________________                                               3,155,683     3,386,954                                                       3,274,214     3,422,119.                                               ______________________________________                                    

More recently, cyclic polycarbonate oligomer mixtures have been preparedand converted to linear polycarbonates, often of very high molecularweight, by contact with a wide variety of polycarbonate formationcatalysts. Reference is made, for example, to U.S. Pat. Nos. 4,644,053and 4,740,583, the disclosures of which are incorporated by referenceherein.

Many of the catalysts used to convert cyclic polycarbonates to linearpolymers have relatively rapid action. This is, of course, oftendesirable, since there is frequently nothing gained by prolonging thepolymerization reaction. However, under certain circumstances a muchslower rate is desired.

An example of such a circumstance is the resin transfer molding process.It involves transfer of the material to be polymerized from a heatedvessel into a mold which contains a reinforcing material, typically afiber mat. In order to facilitate transfer and thorough wetting of themat, the material must be maintained liquid and at a relatively lowviscosity, which requires high temperatures. It is highly desirable forpolymerization to proceed very slowly so that transfer and fiber wettingare optimized.

The present invention provides a method for polymerizing cyclicpolycarbonates which employs a very slow catalyst, facilitating resintransfer molding and similar operations. Said method employs, in manyinstances, catalysts which also function as dyes, affording coloredproducts. The invention also provides polymerizable compositions havingthe same advantages.

In one of its aspects, therefore, the invention includes a method forpreparing a resinous composition which comprises contacting, at atemperature in the range of about 200°-350° C., at least one cyclicpolycarbonate oligomer with a catalytic amount of at least onezwitterionic compound containing positively charged nitrogen andnegatively charged oxygen.

The cyclic polycarbonate oligomer compositions useful in the method ofthis invention generally comprise a plurality of structural units of theformula ##STR1## at least about 60% of the total number of R values aredivalent aromatic organic radicals, the balance thereof being aliphatic,alicyclic or aromatic organic radicals. Such compositions includedimers, trimers and tetramers, as well as cyclic polycarbonate oligomermixtures.

The R values may be different but are usually the same, and may bealiphatic, alicyclic, aromatic or mixed; those which are aliphatic oralicyclic generally contain up to about 8 carbon atoms. Suitable Rvalues include ethylene, propylene, trimethylene, tetramethylene,hexamethylene, dodecamethylene, 1,4-(2-butenylene),1,10-(2-ethyldecylene), 1,3-cyclopentylene, 1,3-cyclohexylene,1,4-cyclohexylene, m-phenylene, p-phenylene, 4,4'-biphenylene,2,2-bis(4-phenylene)propane, benzene-1,4-dimethylene (which is a vinylogof the ethylene radical and has similar properties) and similar radicalssuch as those which correspond to the dihydroxy compounds disclosed byname or formula (generic or specific) in U.S. Pat. No. 4,217,438, thedisclosure of which is incorporated by reference herein. Also includedare radicals containing non-hydrocarbon moieties. These may besubstituents such as chloro, nitro, alkoxy and the like, and alsolinking radicals such as thio, sulfoxy, sulfone, ester, amide, ether andcarbonyl. Most often, however, all R radicals are hydrocarbon radicals.

Preferably at least about 80% of the total number of R values in thecyclic oligomer mixtures, and most desirable all of said R values, arearomatic. The aromatic R radicals preferably have the formula

    --A.sup.1 --Y--A.sup.2 --,                                 (II)

wherein each of A¹ and A² is a monocyclic divalent aromatic radical andY is a bridging radical in which one or two atoms separate A¹ from A².The free valence bonds in formula II are usually in the meta or parapositions of A¹ and A² in relation to Y.

In formula II, the A¹ and A² values may be unsubstituted phenylene orsubstituted derivatives thereof, illustrative substituents (one or more)being alkyl, alkenyl, halo (especially chloro and/or bromo), nitro,alkoxy and the like. Unsubstituted phenylene radicals are preferred.Both A¹ and A² are preferably p-phenylene, although both may be o- orm-phenylene or one o- or m-phenylene and the other p-phenylene.

The bridging radical, Y, is one in which one or two atoms, preferablyone, separate A¹ from A². It is most often a hydrocarbon radical andparticularly a saturated radical such as methylene, cyclohexylmethylene,2-[2.2.1]-bicycloheptylmethylene, ethylene, isopropylidene,neopentylidene, cyclohexylidene, cyclopentadecylidene, cyclododecylideneor adamantylidene, especially a gem-alkylene (alkylidene) radical. Alsoincluded, however, are unsaturated radicals and radicals which containatoms other than carbon and hydrogen; for example,2,2-dichloroethylidene, carbonyl, phthalidylidene, oxy, thio, sulfoxyand sulfone. For reasons of availability and particular suitability forthe purposes of this invention, the preferred radical of formula II isthe 2,2-bis(4-phenylene)propane radical, which is derived from bisphenolA and in which Y is isopropylidene and A¹ and A² are each p-phenylene.

The cyclic oligomer mixtures consist essentially of oligomers havingdegrees of polymerization from 2 to about 30 and preferably to about 20,with a major proportion being up to about 12 and a still largerproportion up to about 15. Since they are mixtures of oligomers havingvarying degrees of polymerization, these compositions have relativelylow melting points as compared to single compounds such as thecorresponding cyclic trimer. The cyclic oligomer mixtures are generallyliquid at temperatures above 300° C. and most often at temperaturesabove 225° C.

The cyclic oligomer mixtures contain very low proportions of linearoligomers. In general, no more than about 5% of such linear oligomers,if any, are present.

The mixtures also usually contain low percentages, if any, of polymers(linear or cyclic) having a degree of polymerization greater than about30. For the purposes of this invention, which often require relativelyvery low viscosities at the temperatures at which the oligomers areliquid, it is usually preferred and sometimes essential for theproportion of such polymers having a high degree of polymerization to below, generally no higher than about 5%. Removal of such polymers, whennecessary, is easily achieved by precipitation with a non-solvent, asdisclosed, for example, in the aforementioned U.S. Pat. No. 4,644,053.

According to the present invention, conversion of the cyclicpolycarbonates to linear polycarbonates is effected by contact with atleast one zwitterionic compound containing positively charged nitrogenand negatively charged oxygen. The charges in said compound may be fullor partial charges.

Full charges are present, for example, in tertiary amine oxides such aspyridine N-oxide. Partial charges are present in various highlyconjugated compounds in which a tautomeric structure withelectropositive nitrogen and electronegative oxygen exists; saidcompounds frequently also function as dyes, forming coloredpolycarbonates when employed in the method of this invention. Examplesof such highly conjugated compounds are the spiroindoline pyran of theformula ##STR2## and the stilbazonium compound of the formula ##STR3##which have the respective tautomeric structures ##STR4## and ##STR5##

The polymerization reaction is typically conducted by merely contactingthe cyclic oligomer mixture with the catalyst at a temperature in therange of about 200°-350° C., preferably about 200°-300° C., untilpolymerization has proceeded to the extent desired. Although a solventmay be used, it is not necessary and is frequently not preferred. It iswithin the scope of the invention to conduct the polymerization in amold to produce a molded article, or in an extruder to produce a linearpolycarbonate as the extrudate.

Compositions comprising cyclic oligomers which comprise units of formulaI and at least one zwitterionic compound as described hereinabove may beprepared by dissolving the cyclic compositions in a suitable solvent,preferably methylene chloride to which said zwitterionic compound isadded in the desired proportions. The solvent is then evaporated toreduce an intimate blend of said zwitterionic compound with the cycliccomposition. Such blend is stable at ambient temperatures but may beslowly polymerized by heating to an appropriate temperature.Polymerizable compositions of this type are another aspect of theinvention.

The proportion of catalyst used in the method of this invention willdepend to some extent on the molecular weight of the polymer desired andthe time available for completion of the polymerization reaction. Sincea "living" polymerization is involved, the molecular weight of thepolymer will vary inversely with the proportion of catalyst used. On theother hand, the reaction rate varies directly with the proportion ofcatalyst. Therefore, as said proportion is increased, the time requiredfor polymerization and the molecular weight of the product bothdecrease. Balancing these factors, it is generally found that catalystproportions of about 0.001-0.5 mole percent, based on structural unitsin the oligomer, are satisfactory.

The invention is illustrated by the following examples. The cyclicpolycarbonate composition used in each example was a mixture ofbisphenol A cyclic polycarbonate oligomers, principally having degreesof polymerization from 2 to about 6 and substantially free from linearpolycarbonate; molar proportions thereof are in terms of carbonateunits. Molecular weights were determined by gel permeationchromatography relative to polystyrene.

EXAMPLES 1-3

A series of polymerization experiments was run at 280° C. in a reactionvessel fitted with a stirrer maintained at a constant speed of 100 rpm.,by means of a motor which compensated for increasing viscosity byproportionally and measurably increasing the current. By means of acomputer, plots were made of viscosity (as determined from currentinput) against time and the slopes of the resulting curves at the pointsof maximum rate of change of viscosity were determined. These slopeswere in turn converted to relative polymerization rates, using lithiumphenoxide as a standard having a value of 1.00. Since the viscosities atthe points of maximum rate of change corresponded to a polymer level ofonly about 35-40% high molecular weight material, only the early part ofeach reaction was considered in this portion of the experiment.

In each example, a 50-gram portion of cyclic polycarbonate oligomers washeated and stirred for a period of time sufficient to establish that nopolymerization was taking place. A sample of zwitterionic compound inthe amount of 0.1 mole percent was then added and stirring was continuedas the viscosity data were taken and until the reaction was complete.The weight and number average molecular weights of the resulting linearpolycarbonates were then determined. The results are given in thefollowing table.

    ______________________________________                                                             Relative             Mw/                                 Example                                                                              Catalyst      rate     Mw    Mn    Mn                                  ______________________________________                                        1      Pyridine N-oxide                                                                            0.03     41,700                                                                              25,100                                                                              1.7                                 2      Spiroindoline pyran,                                                                        0.02     40,100                                                                              22,000                                                                              1.8                                        formula III                                                            3      Stilbazolium  0.47     49,600                                                                              28,500                                                                              1.7                                        compound,                                                                     formula IV                                                             ______________________________________                                    

What is claimed is:
 1. A method for preparing a resinous compositionwhich comprises contacting, at a temperature in the range of about200°-350° C., at least one cyclic polycarbonate oligomer with acatalytic amount of at least one zwitterionic compound containingpositively charged nitrogen and negatively charged oxygen.
 2. A methodaccording to claim 1 wherein the zwitterionic compound is a tertiaryamine oxide or has a tautomeric structure with electropositive nitrogenand electronegative oxygen.
 3. A method according to claim 1 wherein thecyclic polycarbonates comprise a plurality of structural units of theformula ##STR6## wherein at least about 60% of the total number of Rvalues are divalent aromatic organic radicals, the balance thereof beingaliphatic, alicyclic or aromatic organic radicals.
 4. A method accordingto claim 3 wherein each R radical has the formula

    --A.sup.1 --Y--A.sup.2 --,                                 (II)

wherein each of A¹ and A² is a monocyclic divalent aromatic radical andY is a bridging radical in which one or two atoms separate A¹ from A².5. A method according to claim 4 wherein the zwitterionic compound ispresent in the amount of about 0.001-0.5 mole percent, based onstructural units in the oligomer composition.
 6. A method according toclaim 5 wherein A¹ and A² are each p-phenylene and Y is isopropylidene.7. A method according to claim 6 wherein the zwitterionic compound ispyridine N-oxide.
 8. A method according to claim 6 wherein thezwitterionic compound has a tautomeric structure of the formula ##STR7##9. A method according to claim 6 wherein the zwitterionic compound has atautomeric structure of the formula ##STR8##
 10. A polymerizablecomposition comprising cyclic polycarbonate oligomers comprisingstructural units of the formula ##STR9## wherein at least about 60% ofthe total number of R values are divalent aromatic organic radicals, thebalance thereof being aliphatic, alicyclic or aromatic organic radicals;and at least one zwitterionic compound containing positively chargednitrogen and negatively charged oxygen.
 11. A composition according toclaim 10 wherein each R radical has the formula

    --A.sup.1 --Y--A.sup.2 --,                                 (II)

wherein each of A¹ and A² is a monocyclic divalent aromatic radical andY is a bridging radical in which one or two atoms separate A¹ from A².12. A composition according to claim 11 wherein the zwitterioniccompound is present in the amount of about 0.001-0.5 mole percent, basedon structural units in the oligomer composition.
 13. A compositionaccording to claim 12 wherein A¹ and A³ are each p-phenylene and Y isisopropylidene.
 14. A composition according to claim 13 wherein thezwitterionic compound is pyridine N-oxide.
 15. A composition accordingto claim 13 wherein the zwitterionic compound has a tautomeric structureof the formula ##STR10##
 16. A composition according to claim 13 whereinthe zwitterionic compound has a tautomeric structure of the formula##STR11##